With the advent of micro-machining technology, microfluidic devices have proliferated (for example, U.S. Pat. No. 5,637,469 to Wilding et al., U.S. Pat. No. 4,983,038 to Ohki et al., U.S. Pat. No. 4,963,498 to Hillman et al., U.S. Pat. No. 5,250,263 to Manz et al., U.S. Pat. No. 5,376,252 to Ekstrom et al., E.P. Patent Publication 0381501B1, and Petersen, E. (1982) Proc. of the IEEE, vol. 70, No. 5, pp. 420-457). A practical limitation for particle-containing liquids such as blood is the sedimentation of particles within the device. Following loading the liquid in the device, appreciable particle sedimentation can occur within the time required to position the device in a measurement apparatus. For example, if the sample flow is slowed or stopped, blood cells can measurably settle out of plasma within 20 seconds. Without a sample management method and apparatus for sedimentation mitigation, quantitative analysis, especially using more than one analysis method sequentially, is impractical. Moreover, if samples are first collected and then transported to a measurement apparatus, as in a clinical setting or in field sampling, particle sedimentation can make accurate analysis impossible.
Microfluidic devices having sample storage reservoirs are known in the art (for example, E.P. Patent Publication 0381501B1). Because of particle sedimentation, these devices are useful only for samples without particles. Flow cytometric microfluidic devices are also known in the art (for example, U.S. Pat. No. 4,983,038 to Ohki et al.). Flow cytometric measurements are specifically applicable to particle-containing liquids. However, without sedimentation mitigation the measurements can be performed only immediately following sample collection.